Download Pluto, the dwarf planet

Pluto, the dwarf planet
By NASA.gov, adapted by Newsela staff on 11.10.16
Word Count 659
TOP: Pluto nearly fills the frame in this image from the Long Range Reconnaissance Imager (LORRI) aboard NASA's New
Horizons spacecraft, taken on July 13, 2015. MIDDLE: Pluto and Charon are shown in enhanced color in this image.
BOTTOM: In this image from NASA’s New Horizons spacecraft, great blocks of Pluto’s water-ice crust appear jammed
together in the informally named al-Idrisi mountains. Photo: Photo: NASA.
Pluto was discovered in 1930 by an astronomer named Clyde Tombaugh. An astronomer is
a person who studies stars and other objects in space.
At the time, Pluto was considered the ninth planet in the solar system. It was also the
smallest planet. In 2006, astronomers changed their minds about Pluto. They decided it
was too small to be classified as a regular planet.
Now, Pluto is considered a dwarf planet. A dwarf planet orbits the sun just like other
planets, but it is much smaller. Like planets, dwarf planets are roughly spherical, or about
the shape of a ball.
Goes In Circles
Pluto orbits, or circles, the sun in a disc-like zone called the Kuiper Belt. This region is
packed with small, rocky objects like comets and asteroids. It is located beyond the orbit
of Neptune. Like the other icy objects in the Kuiper Belt, Pluto is a very cold world. Its
average temperature is 380 degrees below zero.
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Pluto is very small. It is less than one-fifth the size of Earth and is even smaller than Earth's
moon. Its diameter, which is the distance all the way around its middle, is 1,475 miles.
Pluto most likely has a rocky core, or center. Scientists believe this core is surrounded by a
layer of water ice. The surface of the planet is coated with more unusual kinds of ice.
These ices are not frozen water. Instead, they are made of frozen gases.
A Place Around The Sun
Like Earth, Pluto orbits the sun. Pluto's orbit is much bigger and much farther from the sun
than Earth's. The Earth takes one full year to orbit the sun. It takes Pluto 248 years to
complete its orbit.
Pluto does not travel in a circle around the sun. Instead, it follows a path that is long and
stretched-out, like an oval. The orbit is so long that Pluto is often very distant from the sun.
At those times it is also very distant from Earth. For that reason, it has been very hard to
study.
Pluto's Surface On Thin Ice?
When Pluto gets close to the sun, its surface ices change into gases. The gases then
begin to rise. They form a thin atmosphere, or cloud of gases. For a time, this atmosphere
surrounds the planet.
When Pluto travels far away from the sun it becomes much colder. During this time, most of
the planet's atmosphere freezes. It then falls to Pluto's surface as snow.
Pluto also rotates, or turns, just as the Earth does. One full rotation of a planet is what we
call a day. One full Pluto rotation takes 6.4 Earth days.
Pluto has five moons. The largest is named Charon and is almost half Pluto's size. It takes
Charon just over six Earth days to circle around Pluto. At the same time, Pluto is rotating,
too. One full rotation, or spin, takes Pluto about six Earth days, too.
Pluto's other moons are Nix, Hydra, Kerberos and Styx. They are all farther from Pluto than
Charon is.
Picture This
In 2006 the unmanned New Horizons space probe set off for Pluto. By the spring of 2015,
it arrived above the planet. New Horizons will not land. Instead, it is flying above Pluto,
taking pictures.
Scientists are very puzzled by the pictures New Horizons has sent back. For example,
some of the pictures show large mountains up to 9,800 feet high. Scientists did not think
Pluto would have any mountains. They think the ones they see are made of ice.
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How Pluto Got Its Name
An 11-year-old girl came up with Pluto's name.
In 1930, after scientists discovered Pluto, they asked the public for ideas on what to call it.
Eleven-year-old Venetia Burney, from England, told her grandfather she thought it should
be named after the Roman god Pluto. Venetia's grandfather then told the scientists about
Venetia's idea. They liked the name and decided to use it.
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Quiz
1
2
3
4
Look at the picture at the top of the article. What does it show?
(A)
a close-up picture of Pluto
(B)
a long-distance picture of the moon Charon
(C)
the ice mountains on Pluto's surface
(D)
the first image of the moon Charon taken from Pluto
Based on the caption and the article, what do we know about the formations in the bottom
picture?
(A)
The formations in the picture are not mountains.
(B)
The formations in the picture are made of ice.
(C)
The formations in the picture exist on the moon Charon.
(D)
The formations in the picture were photographed from land.
Which of the following answer choices BEST describes the structure of the introduction
[paragraphs 1-3]?
(A)
chronological
(B)
problem/solution
(C)
comparison
(D)
description
Which paragraph from the section "Pluto's Surface On Thin Ice?" gives information about how
snow forms on Pluto?
This article is available at 5 reading levels at https://newsela.com.
4
Answer Key
1
2
3
4
Look at the picture at the top of the article. What does it show?
(A)
a close-up picture of Pluto
(B)
a long-distance picture of the moon Charon
(C)
the ice mountains on Pluto's surface
(D)
the first image of the moon Charon taken from Pluto
Based on the caption and the article, what do we know about the formations in the bottom
picture?
(A)
The formations in the picture are not mountains.
(B)
The formations in the picture are made of ice.
(C)
The formations in the picture exist on the moon Charon.
(D)
The formations in the picture were photographed from land.
Which of the following answer choices BEST describes the structure of the introduction
[paragraphs 1-3]?
(A)
chronological
(B)
problem/solution
(C)
comparison
(D)
description
Which paragraph from the section "Pluto's Surface On Thin Ice?" gives information about how
snow forms on Pluto?
Paragraph 9:
When Pluto travels far away from the sun it becomes much colder. During this
time, most of the planet's atmosphere freezes. It then falls to Pluto's surface as
snow.
This article is available at 5 reading levels at https://newsela.com.
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